An electrical submersible pump assembly (ESP) for a well typically includes a centrifugal pump driven by a submersible electrical motor. The ESP is normally installed within the well on tubing. Many wells produce a combination of oil and water as well as some gas. Centrifugal pumps are mainly designed to handle liquid and will suffer from head degradation and gas locking in the presence of a high percentages of free gas. Several techniques have been developed to remove the gas before it enters the pump.
One technique relies on causing the well fluid to flow downward before reaching the pump intake to cause separation of gas. Gas bubbles within the well fluid flow tend continue flowing upward as a result of the buoyant force of the gas bubbles. The downward flowing liquid in the well fluid creates an opposing drag force that acts against the upward moving bubbles. If the upward buoyant force is greater than the downward drag force, the bubbles will break free of the downward flowing well fluid and continue moving upward. Buoyancy is a function of the volume of the bubble, and the drag force is a function of the area of the bubble. As the diameter of the bubble increases, the buoyant force will become larger than the drag force, enabling the bubble to more easily separate from the liquid and flow upward. Consequently, if the bubbles can coalesce into larger bubbles, rather than dispersing into smaller bubbles, the separating efficiency would be greater.
A shroud may be mounted around the portions of the ESP to cause a downward flow of well fluid. In one arrangement, the upper end of the shroud is sealed to the ESP above the intake of the pump, and the lower end of the shroud is open. The perforations in the casing are located above the open lower end of the shroud in this arrangement. The well fluid will flow downward from the perforations past the shroud and change directions to flow back up into the shroud, around the motor and into the pump intake. Some gas separation may occur as the well fluid exits the perforations and begins flowing downward.
In an inverted type of shroud, the shroud is sealed to the ESP below the pump intake and above the motor, which extends below the shroud. The inlet of the shroud is at the upper end of the shroud above the pump. The perforations in the casing are below the motor, causing well fluid to flow upward past the motor and shroud and back downward into the open upper end of the shroud. Passive gas separation occurs as the well fluid changes direction to flow downward into the shroud.
Another technique employs a gas separator mounted in the submersible pump assembly between the motor seal section and the pump entrance. The gas separator has an intake for pulling fluids in and a rotating vane component that centrifugally separates the gas from the liquid. The liquid is then directed to the entrance of the pump, and the gas is expelled back into the annulus of the casing. The gas separator provides a well fluid to the pump with a gas content low enough so that it does not degrade the pump performance. The quality of the fluid discharged back into the casing is normally of little concern. In fact, it may have a roughly high liquid content, but the liquid will return back downward to the gas separator intake while the gas would tend to migrate upward in the casing.
Normally, a gas separator would not be incorporated with a shrouded ESP because of the problem of disposing of the gas into the well fluid flowing toward the inlet of the shroud. Gas being discharged into flowing well fluid tends to break up into smaller bubbles and become entrained in the flow. If the shroud inlet is on the lower end, any gas discharged from the gas separator into the shroud annulus would be entrained in the downward flowing fluid and re-enter the inlet. If the shroud inlet is on the upper end, any gas discharged from the gas separator would flow upward through the annulus surrounding the shroud and might fail to separate from the liquid at the inlet of the shroud where the well fluid begins flowing downward.